1. Field of Invention
The present invention relates to polyolefin compositions, and more particularly, to polyolefin compositions comprising an impact modifying fluid.
2. Description of Related Art
In many applications it is possible to replace relatively expensive engineering resins such as acrylonitrile-butadiene-styrene (ABS) with lower cost commodity resins such as polypropylene homopolymer by incorporating various fillers and/or reinforcing materials into the commodity resins. It is well known that incorporating such fillers into commodity resins such as polypropylene homopolymer tends to increase the stiffness of the material, but it also tends to decrease the impact strength of the material. The tradeoff between stiffness and toughness is well known in the art of filled and reinforced polyolefins.
The prior art uses a variety of approaches to increase the impact strength of polyolefins. Polypropylene homopolymer is sometimes blended with about 30% or more by weight of one or more rubbery copolymers comprising at least one alpha olefin such as ethylene and propylene or ethylene and octene, which are typically formed using metallocene catalysts. Blends of polyolefin homopolymers and rubbery copolymers exhibit higher Gardner impact strength than polyolefin homopolymers alone, but there is a significant loss in flexural modulus. Moreover, because rubbery copolymers are relatively expensive and are used in large quantities, this approach to impact modification adds significant cost to the material.
Another prior art approach to impact modification is to add about a 20% by weight loading of calcium carbonate to the molten polypropylene. Polypropylenes modified in this manner can achieve Gardner impact strength values that approach 100 in-lbs while maintaining a flexural modulus of about 250 kpsi. Adding fiberglass to polypropylene can increase the flexural modulus of the material to about 550 kpsi, but the Gardner impact strength of the material is only about 10 in-lbs. Although this approach to impact modification is less expensive than the use of rubbery copolymers, the use of calcium carbonate and/or glass fiber to modify the impact of polyolefins increases the density of the material, and thus the weight of molded parts, which is undesirable in some applications such as in automotive and packaging applications.